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Episode #54 - COVID-19: Mixed and fractional vaccine doses

17 September 2021 | Science conversation

Summary

Many countries are using the approach of mixing vaccine doses from different manufacturers. What do we know about the safety and efficacy of this approach? What does the evidence tell us about using a fraction of the vaccine dose ? Dr Katherine O’Brien explains in Science in 5 this week.

Podcast

Transcript

Vismita Gupta-Smith

We are talking today about mixing vaccine doses and about fractional doses in Science in 5 today. Hello and welcome, I'm Vismita Gupta-Smith, we are talking to Dr Katherine O'Brien. Welcome, Kate.

Let's start with mixing doses from two different manufacturers. Many countries are taking this approach. What does the evidence say so far about the efficacy of this approach?

Dr Katherine O'Brien

This is such an important question because there are over 17 vaccines that are being used around the world now, and most countries have more than one vaccine that's being used in the program. 

So what would be easiest for  countries and for people is to say, you know, one COVID vaccine is like another and I can have one dose from one product and a different dose from another product. But we don't actually have the evidence for most of that mixing and matching.

We do have evidence on both the immunogenicity and the safety of combining an AstraZeneca vaccine dose with either of the mRNA products, the Pfizer, the Moderna products. Non evidence shows that  you get a really good response from an immune perspective and these are safe things to do.

What we know is that it's you get a better response if you start with the AstraZeneca vaccine and then give the mRNA vaccine. But both approaches can work.

Apart from that, we don't yet have the evidence on other mix and match regimens. But those trials are underway, and we're eagerly awaiting to see the results of the trials.

Vismita Gupta-Smith

Kate, speak to us a little more about the safety of this approach.


Dr Katherine O'Brien

Yes, so we have had clinical trials done combining with a first dose, either AstraZeneca or an mRNA vaccine and then the second dose, you know, switching over to the other product. And what we know about the safety is that the amount of reaction in your arm and some of the general short term reactions that people get, you know, not feeling well or a low grade fever are generally speaking the same, whether you mix and match or whether you use the same regimen. So we do have safety evidence on this, and that's an important attribute of anything that we would recommend is to be sure we've looked at the safety already.

Vismita Gupta-Smith

Kate, we're also hearing about fractional doses. Tell us a little more about what that is and what do we know about it so far?

Dr Katherine O'Brien

So the question of fractional doses is it's not unique to COVID vaccines. It also is a topic that we explore for other vaccines. But I'll speak specifically about this issue for COVID vaccines.

You know, the clinical trials that were done for these vaccines did research to figure out what the dose should be that people should receive. And those clinical trials have shown that when you're given those vaccines with those doses, we have very high protection against severe disease. But we know that in the future, there may be some evidence that says that there may be a need for booster doses, or it may be possible for some of the vaccines that we have to reduce the dose and optimize the supply by doing that.

The research to evaluate whether or not what's called a fractional dose. So a fraction of the usual dose, whether that's effective, that research is going on. We don't have the answer to that question yet, but it certainly is an area that we're looking at carefully and we expect that in the future, that evidence will tell us whether or not, if boosters are needed, whether we should use the full dose that you receive as your primary doses, or whether we should be giving a part of that dose a fraction of that dose. 

I'll give you just two other vaccines where this kind of research has helped us in optimizing the vaccine supply and the program. This is the kind of question and kind of research that's been done for polio vaccines and for yellow fever vaccines and others. So this is not a new area of research or a new question that we've never explored with vaccines before. This is something that we do for many vaccines to really optimize the amount of supply that we have and the best immune response with the greatest degree of safety. And so waiting for that research to mature and to become available for now, we don't recommend fractional doses, but we're looking forward to that research coming forward.

 Vismita Gupta-Smith

And as more evidence is generated, we will keep you updated until next time then. Stay safe, stay healthy and stick with science.


 
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